There are vast untapped reserves of organic carbonaceous deposits which have not heretofore been exploited because it has not been economical to do so. The large reserves of oil bearing shale deposits throughout the world are exemplary. One of the largest of these deposits is in the Rocky Mountains of the United States. As the cost of other energy resources increases it becomes economically attractive to extract the oil from such shale.
One of the most attractive methods of extracting oil from oil shale beds is by in situ retorting. In such a technique a chamber is opened and by blasting the chamber is filled with fragmented pieces of oil shale. The top of the resultant bed of shale particles is ignited and air is passed downwardly through the bed to maintain combustion. The resultant heating decomposes the solid or semi-solid organic material (kerogen) in the shale to produce a crude oil, gas, and residual carbon that remains in the shale. Oil descends through the bed, driven by the flow of gas and by gravity. Such oil is collected from the in situ retort and processed into saleable products. U.S. Pat. No. 3,661,423, by Donald E. Garrett describes a process for in situ retorting of oil shale.
The residual carbon in the shale after the oil is retorted burns with the inlet air to supply the heat energy needed for retorting the oil from the shale. Ordinarily a portion of the oil may also burn during in situ retorting.
Suggestion has been made in U.S. Pat. No. 2,780,449 of recovering oil from shale by establishing a narrow combustion wave traveling horizontally between adjacent wells drilled into a shale formation. Oxygen content of the gas fed to the well is controlled to keep the combustion wave nearly coincident with the heat transfer wave and minimize the volume of shale at elevated temperature. A similar technique is proposed in U.S. Pat. No. 2,642,943 for secondary recovery of oil from sands. In both it is deemed desirable to keep the combustion front or combustion wave approximately coincident with the peak temperature of the heat transfer wave. The oxygen content of the driving gas is intermittently changed to permit one or the other of these waves to "catch up" if it lags behind. In U.S. Pat. No. 2,642,943, the wave is indicated to have a thickness measured in inches. No utility is suggested for the off gas from the recovery process.
When the hot gas from the combustion zone passes through the lower retorting zone it picks up combustible light gases such as hydrocarbons, hydrogen and carbon monoxide. The result is an off gas having a heating value of about 25 to 35 BTU/SCF. This off gas should be burned to avoid environmental pollution, but it is difficult to burn this gas because of its inherent low heating value. To burn such gas, additional high heating value gas or even a portion of recovered oil may be added for its additional combustion energy. This is wasteful of not only the additional combustible material but also the heating value of the off gas itself which might otherwise be used for powering operating equipment at the oil shale retorting site.
The amount of residual carbon in the spent shale resulting from retorting is theoretically sufficient to supply the necessary heat for retorting if the oil content of the shale is greater than about 10 gallons per ton. The average oil content of shale presently contemplated for economic production may average about 25 gallons per ton. It is, however, very difficult to prevent the combustion of some part of the oil produced simply by controlling the retort operating parameters since the combustion zone in the retort overlaps the retorting zone. This is due to a substantial extent to the relatively large particle sizes and the consequent limitations on the transport phenomena occurring in the bed of oil shale particles.
It is found that the combustion zone and the zone of hot spent shale in an in situ retort essentially grows continuously throughout the retorting process. That is, the volume above any arbitrary temperature, say 1000.degree. F, grows continuously. As a result if retorting is conducted continously the retort will end up hot for most of its length. It is desirable to utilize the sensible heat contained in the long hot zone of the in situ retort to increase fuel value of the off gas so it can be used to produce power and also to enhance the oil yield.